Thanks to WHoward's great work, it is doable. You are allowed to use the Civ IV Diplomatic Features by using the correct DLL. Please go to WHoward's Pick'N'Mix Mods: http://www.picknmixmods.com/mods/e293a791-74a1-45f5-beda-7921692ecbfe/mod.html
You will find a "Diplomacy - CivIV Features" version of DLL which enables the functionality required by the CivIV Diplomacy Features mod by Putmalk
Thanks Licoln, this is simply awesome.
One more thing: could I possibly make some suggestions about the units' progression? I really love the new units, which fill a huge gap in the rooster, but I think that they are somewhat misplaced in the technology tree. In particular, it is slightly odd to have the cuirassiers and the volley gun, both "Napoleonish" (if not later) side by side with the first gunpowder unit of the game, which basically is a XVIth century musketeer. What I would suggest is to move the cuirassiers and the volley gun to a slightly later technology, maybe along with the cannon. Also, it would be great if you could make some changes to properly represent the 1500-1650 pike&shot warfare - for instance, it would be great to have both the musketeers and the tercio at gunpowder, making the former a ranged unit, i.e. the upgrade of the crossbowmen (more powerful, but with with reduced range 1) and the latter the upgrade of the pikeman. Then the musketeer could upgrade to volley gun, and the tercio to rifleman (or even better, to a new in-between unit, the XVIIIth-century, American revolution-style, "line infantry" - the former American Unique Unit, the Minutemen, would provide the perfect model without any need to create a new one).

I know that fans who make complex suggestions without providing actual help are something of a pain in the a**, but your mod is spectacular, and I would love to see it improved further!
thanks again and bests